This invention relates to freeform fabricated parts and in particular, to a method for modifying the properties of a freeform fabricated part by increasing its density.
As a result of the demand for ways to improve manufacturing efficiency and the need for rapid prototype development, freeform fabrication has become a popular method for manufacturing parts. Freeform fabrication originated with a process called stereolithography wherein a focussed ultra-violet laser scans the top of a bath of a photopolymerizable liquid polymer plastic material, thereby causing the top of the bath and the area just below the surface to polymerize. The polymerized layer is thereafter lowered into the bath and the laser scanning process is repeated until a second polymerized layer is formed. As the second polymerized layer forms, it bonds to the first layer. This process is repeated until a plurality of superimposed layers form the desired part. The shape of the part is first designed in a computer aided design system (i.e., a CAD/CAM system), which is linked to the machine performing the stereolithography process. Most freeform fabrication processes include a computer aided design system for coordinating the execution of the freeform fabrication process. In the case of stereolithography, the laser beam scans the area of the bath necessary to form the freeform fabricated part (hereinafter referred to as xe2x80x9cfreeform partxe2x80x9d) designed on the computer aided design system.
The ability to produce an actual part directly from a design provides many advantages. One advantage includes eliminating the time traditionally used to develop the necessary tooling to manufacture the freeform part. Another advantage includes reducing the amount of machining, such as grinding, milling, drilling, etc., required to complete the part because the freeform fabrication process produces a substantially readily usable final product. Minimizing the amount of hands-on machining, therefore, translates into reducing the amount of potential human error and increasing efficiency. The amount of time saved in preparing for manufacturing also makes the freeform fabrication process attractive for rapid prototype development, which has been one of the main interests surrounding this technology in recent years. The benefit of rapid prototyping includes the ability to manufacture various configurations in a short amount of time, thereby providing designers with actual models of their designs.
Another method of freeform fabrication includes a technique called Three-Dimensional Printing (3DP), which consists of depositing a powdered material (e.g., a powdered ceramic, powdered metal, powdered plastic, or combination thereof) in sequential layers, one on top of the other. After depositing each layer of powdered material, a liquid binder is selectively supplied to the layer of powdered material using a type of ink-jet printing technique in accordance with a computer model of the three-dimensional part being fabricated. Following the sequential application and binding all of the required powder layers, the unbound powder is removed, thereby resulting in the formation of the designed three-dimensional part.
A third method of freeform fabrication includes Selective Laser Sintering (SLS). SLS includes a process whereby a powder dispenser deposits a layer of powdered material into a target area. A laser control mechanism, which typically includes a computer that houses the design, modulates and moves the laser beam to selectively sinter a layer of powder dispensed in the target area. Specifically, the control mechanism operates to selectively sinter only the powder disposed within the defined boundaries of the design. The control mechanism operates the laser to selectively sinter sequential layers of powder, producing a completed part comprising a plurality of layers sintered together yielding the completed design.
A fourth method of freeform fabrication includes Ballistic Particle Manufacturing (BPM). BPM uses an ink-jet printing apparatus wherein an ink-jet stream of liquid polymer or polymer composite material is used to create three-dimensional objects under computer control, similar to the way an ink-jet printer produces two-dimensional graphic printing. The device is formed by printing successive cross-sections, one layer after another, to a target using a cold welding or rapid solidification technique, which causes bonding between the particles and the successive layers.
An additional freeform fabrication technique, includes Fused Deposition Modeling (FDM). FDM consists of building solid objects in a layering fashion from polymer/wax compositions by following the signals produced by a computer aided design system. Specifically, FDM builds structures by extruding a fine filament of plastically deformable material through a small nozzle. The computer aided design system appropriately directs the nozzle over a build surface in the x, y and z directions, thereby creating a three-dimensional object that resembles the design.
Another method of freeform fabrication includes a technique called Photochemical Machining, which uses intersecting laser beams to selectively harden or soften a polymer plastic block. The underlying mechanism used is the photochemical cross-linking or degradation of the material. U.S. Pat. No. 5,490,962 provides a detailed summary of each of the above mentioned freeform fabrication techniques and is hereby incorporated by reference.
The specific processes described above as being suitable for forming products of the present invention are inherently free of fiber reinforcement. In the main, these processes result in the manufacture or production of a freeform based on particulate material. As distinct from the prior art directed to making preforms, articles made by the present invention do not use fiber reinforcement and therefore are fiber free.
The methods described above, however, often result in the fabrication of a porous freeform part, thereby creating undesirable mechanical properties for the freeform part. A freeform part having inadequate strength, unsatisfactory hardness, low temperature tolerance, low abrasion resistance, rough surface finish, poor bonding of individual layers or poor bonding of powder particles within the layers presents a significant limitation to the types of applications in which freeform parts can be utilized. Therefore, what is needed is a means for increasing the mechanical, thermal or other physical properties of freeform parts.
The present invention exploits the porosity of a freeform fabricated part by packing the pores of a freeform part with an infiltrant that is capable of transforming to a ceramic or a ceramic-containing phase. The infiltrant comprises a preceramic polymer, which is selected to bond with the freeform part such that the resulting composition improves the mechanical, thermal and other characteristics of the freeform part. Packing the pores of the freeform part, therefore, increases its density and concomitantly decreases its porosity. Particularly, increasing the density of the freeform part increases one or more or all of the following properties: mechanical strength, hardness, temperature resistance, abrasion resistance, thermal conductivity, and erosion resistance. These properties may be enhanced by carefully fabricating the freeform part such that a certain porosity is imparted, selecting particular infiltrants with various concentrations that add the desired properties to the freeform part, and repeating the infiltration process until the desired density is achieved.
Accordingly, one aspect of the present invention is a process for modifying the properties of a porous freeform part comprising the steps of depositing a porous freeform part in an infiltrant bath, drawing a vacuum on the porous freeform part and the infiltrant bath such that the infiltrant enters the pores within the freeform part, and removing the densified freeform part from the infiltrant bath. The infiltrant is normally a preceramic polymer that is capable of transforming to a ceramic or a ceramic-containing phase. Furthermore, the preceramic polymer is preferably a polymer capable of nanocrystalline ceramic phase growth such that the preceramic polymer can enter the pores within the freeform part. Upon being removed from the infiltrant bath the density of the freeform part increases because the previously empty pores now contain infiltrant, and the infiltrant within the pores of the freeform part transforms to a ceramic or a ceramic-containing phase. Subjecting the freeform part to multiple infiltration processes further decreases the porosity of the freeform part and concomitantly increases its density.
A second embodiment of the present invention includes pressurizing the infiltrant as an alternative to or in conjunction with drawing a vacuum on the porous freeform part and the infiltrant bath such that the infiltrant enters the pores within the freeform part.
A third embodiment of the present invention includes heating the infiltrant while pressure is being applied.
A fourth embodiment of the present invention includes placing the porous freeform part in a vacuum dessicator, applying a vacuum and allowing an infiltrant to enter the vacuum dessicator such that the infiltrant enters the pores within the freeform part.
A fifth embodiment of the present invention includes subjecting the densified freeform part to a series of post-processing steps upon being removed from the infiltrant bath such that the infiltrant within the pores of the densified freeform part transforms to a ceramic or a ceramic-containing phase. The post-processing steps may include one or more or all of the following steps: curing the infiltrant, heating the infiltrant at a rate, duration and temperature such that the infiltrant transforms to a ceramic or ceramic-containing phase within the pores of the freeform article, annealing the transformed infiltrant and cooling the freeform fabricated part and the transformed infiltrant.
A still further embodiment of the present invention includes a freeform fabricated part having pores therein comprising a ceramic or a ceramic-containing phase disposed within a portion of the pores with the freeform part.